Why do you sometimes go through the American Customs checkpoint inside Canada before landing in the U.S.?

Question

A friend of mine (an American citizen) recently flew to Texas from Quebec but had a layover in Toronto. Upon landing at Toronto's airport, he went through American customs...while still in Canada. Why didn't they let him go through this process once he got to Texas? When did Canada agree to let U.S. officials do their checks in Canadian territory? Thanks.

Answer 1

could hvae been canadian customs - AM